Systems or memory layers of the generic type are used, in particular, for medical purposes, in the computer radiography (CR) sector. In them, X-ray images are recorded in a phosphor layer by storing the X-ray radiation passing through an object, for example a patient, as a latent image in the phosphor layer. To read out the stored image, the phosphor layer is irradiated with stimulating light, as a result of which the latter emits, in accordance with the latent image, emission light that is detected by an optical detector and is converted into electrical signals. If required, the electrical signals can be processed further and displayed on a monitor or be outputted on a suitable output appliance, such as, for example, a printer.
In the prior art, systems are known in which the X-ray information items stored in a memory layer are read out with a line scanner. The line scanner comprises an irradiating device for irradiating a linear region of the memory layer with stimulating radiation and a line-shaped detector for positionally resolved detection of the emission radiation excited in the linear region of the memory layer. The memory layer has a multiplicity of individual phosphor particles in which the X-ray information items are stored. The phosphor particles are present in the form of a powder and are embedded in a medium, in particular a plastic. Memory layers of this type are therefore also described as powder image plates (PIPs). As a result of this composition and structure, the stimulating radiation incident on a memory layer is scattered in the memory layer so that not only the primarily to be excited linear region, but also adjacent regions of the layer are excited to emit emission light. The linear region to be read out cannot be sharply limited, which results in a reduction in the sharpness of the image information to be detected.
To reduce this loss of sharpness, memory layers that contain a blue dye were proposed in the prior art. The blue dye partially absorbs the stimulating radiation, which is generally in the red spectral range, in the memory layer and therefore shortens the mean path length that the stimulating radiation traverses in the memory layer. In this way, the linear region to be excited can be better limited on the memory layer thereby increasing the sharpness of the image information items to be read out. However, the absorption of the stimulating radiation simultaneously prevents a deep penetration of the stimulating radiation into the memory layer with the result that only upper sublayers of the memory layer can be excited to emit emission light. Owing to this reduced reading-out depth, the intensity of the emission radiation emitted is reduced, which results in increased signal-to-noise. Despite improved image sharpness, this does not altogether significantly improve the image information.
In addition, the blue dye in the memory layer results only in an increase in the sharpness perpendicular to the extension of the excited linear region on the memory layer. Since the total linear region is irradiated with stimulating radiation, on the other hand, no increase in sharpness is possible in the direction of said region.